As technology advances, high speed integrated circuits chips (IC's) operating at ultra-high or microwave frequencies are adopted in an increasing number of applications. For IC packages, printed circuit board (PCB) assemblies or multi-chip module (MCM)assemblies incorporating such high speed integrated circuit chips, signal leads and interconnecting traces of the packages and assemblies are properly considered to be transmission lines, rather than simply modeled as lumped capacitors. Each high speed integrated circuit chip includes pad driver circuitry for driving these transmission lines.
To avoid impedance mis-match and signal reflections, transmission lines should be terminated with resistance substantially equal to a characteristic impedance (Z.sub.0) of the transmission line. A transmission line can be terminated at either end or both ends of the transmission line. A termination method that results in lowest power dissipation includes terminating an input end of the transmission line, otherwise known as a source termination method. Accordingly, in a high speed integrated circuit chip application, it is preferred that an output impedance of the pad driver is substantially equal to the characteristic impedance of the transmission line.
A difficulty in previously known pad drivers included in integrated circuits is that although the pad drivers are designed with a desired output impedance, an actual output impedance of the pad drivers will change with IC manufacturing process variations, IC power supply variations, signal voltage variations, and temperature variations. Furthermore, while there may be some advantages to packaging the IC's in various different styles of IC packages, difficulties arise in that different styles of IC packages have different characteristic impedances (Z.sub.0), resulting in the undesired impedance mis-match and signal reflections in some of the packages. Although an output impedance of the pad driver may be controlled using a transistor impedance, a gate length shrink can change the impedance value. Gate length shrinks are commonly done when IC manufacturing is migrated to a more advanced process having a smaller IC feature size.
Of course, manual adjustment schemes or automated off chip control schemes, for example using scan control, can be employed to correct pad driver impedance of the integrated circuit chips. However such schemes are inconvenient because they require that complex electronic equipment external to the chip be provided and operated.
What is needed is a flexible and efficient integrated circuit chip that includes pad drivers for driving an impedance of a transmission line, wherein the integrated circuit further includes all additional circuitry to automatically adjust a matching impedance of the pad drivers.